On the Action of the Rays of the Solar Spectrum on Vegetable Colours.

393

rimental processes employed in his investigation ; and points out
several circumstances which require to be attended to in order to
ensure success.

June 16, 1842.
SIR JOHN W. LUBBOCK, Bart, V.P. and Treas., in the Chair.

The following papers were read, viz. —

1 . ie On the Action of the Rays of the Solar Spectrum on Vegetable
Colours." By Sir John Frederick William Herschel, Bart., K.H.,
F.R.S.

The author, having prosecuted the inquiry, the first steps of which
he communicated in a paper read to the Royal Society in February
1 840, relating to the effects of the solar spectrum on the colouring
matter of the Viola tricolor, and on the resin of guaiacum, re-
lates, in the present paper, the results of an extensive series of simi-
lar experiments, both on those substances, and also on a great number
of vegetable colours, derived from the petals of flowers, and the leaves
of various plants. In the case of the destruction of colour of the pre-
parations of guaiacum, which takes place by the action of heat, as
well as by the more refrangible rays of light, he ascertained that
although the non-luminous thermic rays produce an effect, in as far
as they communicate heat, they are yet incapable of effecting that
peculiar chemical change which other rays, much less copiously en-
dowed with heating power, produce in the same experiment. He
also found that the discoloration produced by the less refrangible
rays is much accelerated by the application of artificial terrestrial
heat, whether communicated by conduction or by radiation ; while,
on the other hand, it is in no degree promoted by the purely ther-
mic rays beyond the spectrum, acting under precisely similar cir-
cumstances, and in an equal degree of condensation. The author
proceeds to describe, in great detail, the photographic effects pro-
duced on papers coloured by various vegetable juices, and after-
wards washed with solutions of particular salts ; and gives a minute
account of the manipulations he employed for the purpose of im-
parting to paper the greatest degree of sensitiveness to the action of
solar light. This action he found to be exceedingly various, both as
regards its total intensity and the distribution of the active rays over
the spectrum. He observed, however, that the following peculiar-
ities obtain almost universally in the species of action exerted.

First, the action is positive ; that is to say, light destroys colour,
either totally, or leaving a residual tint, on which it has no further,
or a very much slower action ; thus effecting a sort of chromatic ana-
lysis, in which two distinct elements of colour are separated, by de-
stroying the one and leaving the other outstanding. The older the
paper, or the tincture with which it is stained, the greater is the
amount of this residual tint.

394

Secondly, the action of the spectrum is confined, or nearly so, to
the region of it occupied by the luminous rays, as contra-distinguished
both from the so-called chemical rays beyond the violet, (which act
with chief energy on argentine compounds, but are here for the
most part ineffective.) on the one hand, and on the other, from the
thermic rays beyond the red, which appear to be totally ineffective.
Indeed, the author has not hitherto met with any instance of the
extension of this description of photographic action on vegetable
colours beyond, or even quite up to the extreme red.

Besides these, the author also observed that the rays which are
effective in destroying a given tint, are, in a great many cases, those
whose union produces a colour complementary to the tint destroyed,
or at least one belonging to that class of colours to which such com-
plementary tint may be referred. Yellows tending towards orange,
for example, are destroyed with more energy by the blue rays ; blues
by the red, orange and yellow rays ; purples and pinks by yellow
and green rays. These phenomena may be regarded as separating
the luminous rays by a broadly defined line of chemical distinction
from the non-luminous ; but whether they act as such, or in virtue
of some peculiar chemical quality of the heat which accompanies
them as heat, is a point which the author considers his experiments
on guaiacum as leaving rather equivocal. In the latter alternative,
he observes, chemists must henceforward recognise, in heat from
different sources, differences not simply of intensity, but also of
quality ; that is to say, not merely as regards the strictly chemical
changes it is capable of effecting in ingredients subjected to its in-
fluence.

One of the most remarkable results of this inquiry has been the
discovery of a process, circumstantially described by the author, by
which paper washed over with a solution of ammonio-citrate of iron,
dried, and then washed over with a solution of ferro-sesquicyanuret
of potassium, is rendered capable of receiving with great rapidity a
photographic image, which, from being originally faint and some-
times scarcely perceptible, is immediately called forth on being
washed over with a neutral solution of gold. The picture does not
at once acquire its full intensity, but darkens with great rapidity up
to a certain point, when the resulting photograph attains a sharpness
and perfection of detail which nothing can surpass. To this pro-
cess the author applies the name of Chrysotype, to recall to mind its
analogy with the Calotype process of Mr. Talbot, to which in its
general effect it affords so close a parallel.

2. " Experimental Researches on the Elliptic Polarization of
Light." By the Rev. Baden Powell, M.A., F.R.S., Savilian Pro-
fessor of Geometry in the University of Oxford.

This paper contains an experimental investigation of the pheno-
mena of elliptic polarization resulting from the reflexion of polarized
light from metallic surfaces, and the theory on which they are ex-
plicable ; the analytical results being given in a tabular form, and
applied to the cases of the experiments themselves.